Generation Time Calculator
How many generations fit into a span of years for a species with a given generation time — and how a per-generation rate of change compounds over that span.
Generation time is the average age of parents at reproduction — about 25 to 30 years for humans, a year for many rodents, twenty minutes for a fast bacterium.
How the generation time calculator works
Generation time is the average age of parents at reproduction — about 25 to 30 years for humans, a year for many rodents, twenty minutes for a fast bacterium. Divide any time span by it and you have the generations that have passed.
Selection and mutation act per generation, so the same century is four human generations and two and a half million bacterial ones. That is the whole reason bacteria evolve resistance in years and elephants do not.
Formula: generations = years / generation time; change = (1 + rate)^generations
Worked examples
| Inputs | Generations in the span | Note |
|---|---|---|
| Humans over 10,000 years | 370.37 | 370 generations |
| A bacterium over one year | 26,298 | over 26,000 |
| Fruit flies over a decade | 304.38 | about 300 |
FAQFrequently asked questions
What is generation time?
The average interval between a parent's birth and its offspring's — about 27 years for modern humans, a year for mice, twenty minutes for E. coli in ideal conditions.
Why does it matter for evolution?
Because selection acts once per generation. The same year is one generation for a mouse and thousands for a bacterium, so bacteria adapt thousands of times faster.
How many human generations since agriculture?
About 370 to 400 over ten thousand years — few enough that most of our genome has barely changed since.
What rate of change per generation is realistic?
Strong selection can shift a trait by a per cent or more per generation; typical rates are far lower. Even 0.01% compounds enormously over a million generations.
Does generation time change?
Yes — human generation time has lengthened as first births moved later, and it differs between populations and eras. Use the figure for the population you mean.
Where these figures come from
- NCBI Bookshelf — Molecular Biology of the Cell — growth kinetics and molecular conventions
- CSIRO — Australia's national science agency
Last checked: September 2026. Formulas are the standard textbook forms; assumptions are stated on each page because they are where these models break.